Two general approaches are used for introduction of new genetic information (“transformation”) into cells. One approach is to introduce the new genetic information as part of another DNA molecule, referred to as an “episomal vector,” or “minichromosome”, which can be maintained as an independent unit (an episome) apart from the host chromosomal DNA molecule(s). Episomal vectors contain all the necessary DNA sequence elements required for DNA replication and maintenance of the vector within the cell. Many episomal vectors are available for use in bacterial cells (for example, see Maniatis et al., 1982). However, only a few episomal vectors that function in higher eukaryotic cells have been developed. Higher eukaryotic episomal vectors were primarily based on naturally occurring viruses. In higher plant systems gemini viruses are double-stranded DNA viruses that replicate through a double-stranded intermediate upon which an episomal vector could be based, although the gemini virus is limited to an approximately 800 bp insert. Although an episomal plant vector based on the Cauliflower Mosaic Virus has been developed, its capacity to carry new genetic information also is limited (Brisson et al., 1984).
The other general method of genetic transformation involves integration of introduced DNA sequences into the recipient cell's chromosomes, permitting the new information to be replicated and partitioned to the cell's progeny as a part of the natural chromosomes. The introduced DNA usually is broken and joined together in various combinations before it is integrated at random sites into the cell's chromosome (see, for example Wigler et al., 1977). Common problems with this procedure are the rearrangement of introduced DNA sequences and unpredictable levels of expression due to the location of the transgene in the genome or so called “position effect variation” (Shingo et al., 1986). Further, unlike episomal DNA, integrated DNA cannot normally be precisely removed. A more refined form of integrative transformation can be achieved by exploiting naturally occurring viruses that integrate into the host's chromosomes as part of their life cycle, such as retroviruses (see Cepko et al., 1984). In mouse, homologous integration has recently become common, although it is significantly more difficult to use in plants (Lam et al. 1996).
The most common genetic transformation method used in higher plants is based on the transfer of bacterial DNA into plant chromosomes that occurs during infection by the phytopathogenic soil bacterium Agrobacterium (see Nester et al., 1984). By substituting genes of interest for the naturally transferred bacterial sequences (called T-DNA), investigators have been able to introduce new DNA into plant cells. However, even this more “refined” integrative transformation system is limited in three major ways. First, DNA sequences introduced into plant cells using the Agrobacterium T-DNA system are frequently rearranged (see Jones et al., 1987). Second, the expression of the introduced DNA sequences varies between individual transformants (see Jones et al., 1985). This variability is presumably caused by rearranged sequences and the influence of surrounding sequences in the plant chromosome (i.e., position effects), as well as methylation of the transgene. A third drawback of the Agrobacterium T-DNA system is the reliance on a “gene addition” mechanism: the new genetic information is added to the genome (i.e., all the genetic information a cell possesses) but does not replace information already present in the genome.
One attractive alternative to commonly used methods of transformation is the use of an artificial chromosome. Artificial chromosomes are man-made linear or circular DNA molecules constructed from cis-acting DNA sequence elements that provide replication and partitioning of the constructed chromosomes (see Murray et al., 1983). Desired elements include: (1) Autonomous Replication Sequences (ARS) (these have properties of replication origins, which are the sites for initiation of DNA replication), (2) Centromeres (site of kinetochore assembly and responsible for proper distribution of replicated chromosomes at mitosis or meiosis), and (3) if the chromosome is linear, telomeres (specialized DNA structures at the ends of linear chromosomes that function to stabilize the ends and facilitate the complete replication of the extreme termini of the DNA molecule).
The essential chromosomal elements for construction of artificial chromosomes have been precisely characterized in lower eukaryotic species, and more recently in mouse and human. ARSs have been isolated from unicellular fungi, including Saccharomyces cerevisiae (brewer's yeast) and Schizosaccharomyces pombe (see Stinchcomb et al., 1979 and Hsiao et al., 1979). An ARS behaves like a replication origin allowing DNA molecules that contain the ARS to be replicated as an episome after introduction into the cell nuclei of these fungi. DNA molecules containing these sequences replicate, but in the absence of a centromere they are partitioned randomly into daughter cells.
Artificial chromosomes have been constructed in yeast using the three cloned essential chromosomal elements. Murray et al., 1983, disclose a cloning system based on the in vitro construction of linear DNA molecules that can be transformed into yeast, where they are maintained as artificial chromosomes. These yeast artificial chromosomes (YACs) contain cloned genes, origins of replication, centromeres and telomeres and are segregated in daughter cells with high fidelity when the YAC is at least 100 kB in length. Smaller CEN-containing vectors may be stably segregated, however, when in circular form.
None of the essential components identified in unicellular organisms, however, function in higher eukaryotic systems. For example, a yeast CEN sequence will not confer stable inheritance upon vectors transformed into higher eukaryotes. While such DNA fragments can be readily introduced, they do not stably exist as episomes in the host cell. This has seriously hampered efforts to produce artificial chromosomes in higher organisms.
In one case, a plant artificial chromosome was discussed (Richards et al., U.S. Pat. No. 5,270,201). However, this vector was based on plant telomeres, as a functional plant centromere was not disclosed. While telomeres are important in maintaining the stability of chromosomal termini, they do not encode the information needed to ensure stable inheritance of an artificial chromosome. It is well documented that centromere function is crucial for stable chromosomal inheritance in almost all eukaryotic organisms (reviewed in Nicklas 1988). For example, broken chromosomes that lack a centromere (acentric chromosomes) are rapidly lost from cell lines, while fragments that have a centromere are faithfully segregated. The centromere accomplishes this by attaching, via centromere binding proteins, to the spindle fibers during mitosis and meiosis, thus ensuring proper gene segregation during cell divisions.
In contrast to the detailed studies done in S. cerevisiae and S. pombe, less is known about the molecular structure of functional centromeric DNA of higher eukaryotes. Ultrastructural studies indicate that higher eukaryotic kinetochores, which are specialized complexes of proteins that form on the centromere during late prophase, are large structures (mammalian kinetochore plates are approximately 0.3 μm in diameter) which possess multiple microtubule attachment sites (reviewed in Rieder, 1982). It is therefore possible that the centromeric DNA regions of these organisms will be correspondingly large, although the minimal amount of DNA necessary for centromere function may be much smaller.
The above studies have been useful in elucidating the structure and function of centromeres-. The extensive literature indicating both the necessity of centromeres for stable inheritance of chromosomes, and the non-functionality of yeast centromeres in higher organisms, demonstrate that cloning of a functional centromere from a higher eukaryote is a necessary first step in the production of artificial chromosomes suitable for use in higher plants and animals. The production of artificial chromosomes with centromeres which function in higher eukaryotes would overcome many of the problems associated with the prior art and represent a significant breakthrough in biotechnology research.